The invention relates to cellular network systems with mobile stations communicating with base stations using frequency-hopping, and having an enhanced effect of interference diversity. The cellular-network systems according to the invention may conform to the GSM system known per se, for instance.
New requirements arise when mobile phone services are offered in urban population centres: the system is supposed to have a large capacity, base station equipment should be small-sized because of the very limited possibilities of providing equipment rooms in urban areas, and the system should operate on low radio power levels to make small-sized subscriber's units possible. Moreover, the system is expected to cover also the interiors of buildings, and to offer a good coverage in spite of the fact that it is difficult to predict the radio coverage of antennas to be positioned at low elevations. In addition to the above, the system is supposed to offer easier frequency planning of a radio network than before. Such cellular networks are built of so-called short-range microcells.
To build a mobile phone network in an environment as described above is a task very different from realizing a classic cellular network. While the primary object of a cellular network is to minimize the number of base station sites by increasing the range of the cells to the technical maximum and by installing the maximum number of transceiver units permitted by the frequency band of the network on each base station site, the primary object of microcellular networks is to achieve low radio powers and small installation units suitable for outdoor deployment.
However, conventional cellular technique (cell splitting) does not meet the requirements of a microcellular network. This is due to the fact that with the cell radius decreasing and the radio coverage at the same time becoming more difficult to predict than before (because antennas are positioned below roof-tops to restrict the cell size) either the overlapping of the cells increases or the coverage of the system is compromised (depending on a shadowing caused by buildings or natural obstacles in each direction). An increasing degree of overlapping of the cells makes it necessary to widen the reuse pattern, which eliminates at least part of the additional capacity supposed to be obtained by means of cell splitting.